The Android GDB (JimDB) Wiki and pre-built binaries now include instructions and support for Android on x86. Not only is it useful on the growing number of x86 devices, it is also useful on the x86 emulator included in the Android SDK (for developers without access to a device). Presently, the x86 version is entirely separate form the ARM version - you would need both versions in different directories if you have devices on both platforms.
The updated GDB also has limited support for on-demand decompression on Android. If you've been noticing random seg faults when debugging Fennec, the new version will ignore these seg faults. See "Random segmentation faults when debugging Fennec" and "monitor set ignore-ondemand" in the Wiki for more information.
Next up, I will be working on offering a B2G version of JimDB with similar functionality and benefits as the Android version. See the dev.b2g posting for the discussion. Let me know what you think!